1. Field of the Invention
The present invention relates to flow control valves. More particularly, the invention relates to throttling valves such as those used in the natural gas pipeline industry, which must be capable of minimally restricting flow therethrough during peak gas flow rates therethrough, and must be further capable of restricting the flow therethrough during periods of lower natural gas demand without causing undesirable noise and/or vibration in the valve.
2. Background of the Art
Flow control valves are used to throttle the flow of fluids, including gases, through pipes and pipelines. The valve typically includes a body into which the opposed ends of the pipe are received, and a valve element received within the body and having a flow passage therethrough which is selectively alignable with the pipe ends to permit, or restrict, flow through the valve. During periods of high pipeline demand, such as the winter months where the pipeline is a natural gas pipeline, the valve element is positioned to provide minimal restriction to gas flow. During nonpeak months, such as during summer where gas demand is low, the valve element is positioned to provide a substantial restriction to the gas flow through the valve.
One problem associated with the throttling of pipeline flows with valves is the occurrence of "noise" which is caused by vibration in the valve and pipe caused by valve induced shear turbulence and vortices formed in the fluid passing through the valve. In a typical ball valve arrangement, the valve element is a spherical member having a cylindrical bore therethrough. The body of the valve likewise includes cylindrical inlet and outlet passages therethrough, which are selectively alignable with the valve element passage by rotating the valve element. When the valve element is rotated to move the valve element passage from a fully aligned position with the inlet and outlet passages in the body to a less aligned position, the fluid flowing through the valve must change direction because the flow path through the valve element passage is at an angle to the flow path through the inlet and outlet passages of the body and the flow passage through the pipe. Additionally, a portion of the valve element body extends into the flow path at the valve inlet and presents a reduced cross-section flow path for the fluid flowing through the valve. This reduced area causes the fluid to accelerate into the valve element while fluid stream loses pressure, i.e., the fluid expands as it enters the valve element. These changes are non-uniform in the flow path through the valve, because portions of the fluid flow are not directly affected by the movement of the ball to alter the flow path through the ball, whereas other portions of the fluid stream are substantially effected. As a result, shear occurs within the fluid stream where high pressure, low velocity portions of the fluid stream contact high velocity, low pressure fluid stream areas through the valve. This shear can induce noise in the valve which can destroy the valve or render it unsuitable for use in noise sensitive areas.